This invention relates to the manufacture of cables and flexible power cords constructed from several twisted elements in general and more particularly to an improved method and apparatus for use in a manufacturing method in which immediately before the cable sheath is applied, so-called SZ-twisting, where the elements are twisted alternatingly by sections with right-hand twist and left-hand twist is employed.
Developments in the field of SZ-twisting technology for electrical cables, which got its start at the beginning of the Sixties, has led to SZ-twisting methods and SZ-twisting machines by means of which two to five elements can be twisted together without problems. SZ-twisting is presently used in combined pair, triplet or spiral four (star quad) basic unit stranding of communications cables as well as in stranding, followed by sheathing of multi-conductor, permanently installed power cables with conductor cross sections up to 2.5 mm.sup.2 (U.S. Pat. No. 3,823,536). As a rule, the SZ-twisting is done by means of rotating longitudinal accumulators, the rotary motion of which is changed at intervals.
Employment of SZ-twisting is of interest wherever a twisting operation and another operation can be combined. In addition to the aforementioned manufacturing operations, this also applies to the twisting of flexible elements in combination with a further operation, for instance, it applies to the twisting and subsequent sheathing of the conductors of flexible power cords, the conductors of which consist of stranded wire. Such flexible cords are, for instance, telephone cords or connecting cords for household appliances. However, SZ-twisting can also be used in the manufacture of optical cables (optical waveguide cables), for instance, in the twisting and subsequent sheathing of high tensile strength wires or filaments for manufacturing a high tensile strength support member.
The present invention is based on the discovery that it is important, particularly in the case of flexible SZ-twisted cords, that the lay (twist) obtained by the twisting not be reduced by subsequent operations and that the reversal points of the twist direction of the mutually SZ-twisted conductors be made as short as possible, so that the flexibility of the cord is not adversely affected. While in the case of SZ-twisting solid wire conductors, untwisting of the twisted assembly, especially in the vicinity of the reversal points of the twist direction, is prevented by influence of tensile stresses, partly by plastic deformation of the conductors, the opposite effect occurs in the case of flexible conductors made of stranded wire, i.e., the torsional stresses present in the twisted material can equalize particularly well in the vicinity of the reversal points of the twist direction. This takes place particularly where the SZ-twisted material is conducted over a long unsupported distance in air after being twisted, as is necessary, for instance, during a subsequent covering with an extruded plastic sheath in the region of the cooling section (water basin) of the extruding apparatus.
For the SZ-twisting elements of electrical cables, equipment is known in which rotating twisting heads cooperate with stationary twisting heads and the frictional connection of the stationary twisting heads is changed at intervals. The stationary twisting heads form so-called torsion blocks, since in the closed condition, they prevent the torsion of the material to be twisted, starting from the rotating twisting head, from affecting sections of the material which are located beyond the stationary twisting head. Such torsion blocks are also known in SZ-pretorsioning of conductors.